In electrophotography and electrostatic printing process, carriers, sleeves, and doctor blades are electrifying members having a function of imparting triboelectric charges to a toner for electrostatic development. The electrifying member is generally coated with a suitable agent for controlling its electric charging.
A typical two-component developer for dry copiers consists of fine toner particles and larger carrier particles. The toner and the carrier are electrostatically charged to opposite polarity by mixing and agitating them in frictional contact. The thus charged toner is electrostatically attached to an electrostatic latent image formed on a photoconductor drum to form a visible image, which is transferred and fixed to a transfer sheet to accomplish duplication. As the carrier, oxidized or unoxidized iron powder is commonly used. When iron powder carrier as such is mixed with the toner to form a developer, the carrier has insufficient triboelectric charging characteristics to the toner, and the toner gradually adheres to the carrier surface to form a toner film (known as toner-spent phenomenon), which raises the problem that the charging characteristics of the carrier change with time and eventually the developer has a shorter life. Another problem is a considerable difference in the charging characteristics of the carrier between dry and humid states.
To overcome these drawbacks, the carrier is often coated on the surface with suitable resins such as acrylic resins, styrene-acryl copolymers, silicone, polyester, and fluoro-resins. Of these, silicone resins and fluoro-resins draw an attention in that they can alleviate the toner-spent phenomenon on account of their low surface energy. In particular, silicone resins can be dissolved in a variety of solvents and prepared to a variety of molecular structures through a choice of monomers. This leads to the advantage that carriers having a wide range of charge quantity level can be formed by a choice of the silicone resin, a third component, and working conditions. For example, it was proposed to control the charging quantity of the carrier by curing a room temperature, moisture curable silicone resin at elevated temperature (JP-B 61948/1987) or by using a silicone resin loaded with a tin compound (JP-B 3181/1990 and 1392/1994). These methods are successful in providing a large charge quantity to the positive chargeable toner, but fail to provide a sufficient charge quantity to the negative chargeable toner.
A number of methods were proposed to solve the above problems. For example, JP-A 127569/1980, 147478/1989, 140951/1986, and 104522/1995 disclose nitrogenous silicone resins and aminosilane-containing silicone resins. JP-A 157751/1980, JP-B 8827/1984, JP-B 23308/1987, JP-A 140358/1981, and JP-A 228261/1985 disclose silicone resins reacted or mixed with organic resins, especially acrylic resins. These resins, however, fail to provide a constant charge quantity and are less durable.
Particularly, since the mixture or reaction product of a silicone resin and an acrylic resin requires compatibility between the resins, the typical silicone resin used is a phenyl-rich silicone resin having good compatibility with organic resins, which is likely to give rise to the toner-spent phenomenon.